I just bought a 2005 Silverado with 160k miles and I am trying to diagnose the AC system. Most of the time the compressor never kicks on and so it blows warm air. Occasionally the compressor does kick on and the air gets cool but the compressor will only run for maybe 5 seconds or less. Sometime it will run a little longer the first time but still less than 30 seconds.

I hooked up gauges to the system and high and low side show equalize to just under 100 psi. I took this as a good sign that at least there isn't a leak. When the compressor kicks on the lowest I have seen the low side is about 55 psi and the high side barely rises and maybe gets to 125 psi. I guess this could be because the compressor isn't running long enough to generate much pressure. I have also heard that this could mean and obstruction in the low side of the system.

I have also tried to jumping the low side pressure switch but that didn't change anything.

What would be the next step, should I evacuate the system and vacuum it down to clear any moisture and try to recharge.

How would you diagnose this further. or are there parts I should just replace like the receiver drier?

Finally I am just trying to understand this AC system. Could you explain how the high and low side work. What I mean is the high and low ports are on the same hard line. From what I understand there is an orfice tube in the middle but I don't know that that does.

  • Sounds like the Freon charge is low or has non condensable gases in the charge, dump the ac charge and evacuate the system then charge with proper amount of Freon.
    – Moab
    Aug 1, 2019 at 19:56
  • Agree with @Moab ... but you'd probably would want to use R134a refrigerant and not Freon (R12). This post might help. Aug 1, 2019 at 22:49

1 Answer 1


The orifice tube is your system's expansion device. It suddenly reduces the liquid refrigerant pressure and tries to meter the liquid refrigerant somewhat (albeit much less accurately than a thermal expansion valve can do) so that expansion can take place inside the evaporator. It's inside the engine bay because they usually make some noise.

55 psi suction sounds too high to mean obstructions, low refrigerant charges and inoperative evaporator blower, while 125 psi sounds too low to mean condenser fan problems... maybe a faulty low pressure clutch cycling switch, low pressure cut out switch or clutch coil? Faulty harness perhaps? Unfortunately diagnosing AC is often hit and miss...

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